With the progress of semiconductor process and the miniaturization of microelectronics devices, the density of semiconductor devices on a wafer is increasing and correspondingly the intervals between the devices are becoming shorter and shorter, which increases the difficulty of the etching process for contact holes.
In prior art methods for forming contact holes, a photo resist layer serves as an etching mask for etching the underlying interlayer dielectric (ILD) layer. Especially for the 45 nm process, a pitch (i.e., a distance of centers of two neighboring contact holes) for contact holes must be lower than 155 nm and the “after development inspect critical dimension” (ADICD) must be around 70-80 nm. As far as the current lithographic tools of single-exposure patterning (SP) are concerned, it is impossible to create contact holes with pitch lower than 155 nm in one exposure.
In order to solve the above-mentioned problem, the current solution is to use a double-exposure patterning technology to create desired contact holes. For example, a lithography-lithography-etching (LLE) process is used, in which two exposures with two photo masks are performed on a photo resist layer and then followed by one etching step, so as to form the desired contact holes in the underlying ILD layer. Instead, a lithography-etching-lithography-etching (LELE) process can be used, in which two photo masks are respectively applied to two photo resist layers for two exposures and two etchings to transfer the photo resist patterns to a hard mask layer and then followed by one etching step, so as to form the desired contact holes in the underlying ILD layer.
However, since after developing of photo resist in the second photolithography process, the photo resist possibly is needed to be stripped off for rework due to non-uniformity of critical dimension or defocus, which would damage the pattern formed on the photo resist or hard mask by the first developing (or etching) of photo resist, so that the yield of subsequent process is degraded.
Accordingly, there is a need to provide a method for forming contact holes immune to the influence of photo resist rework, so as to improve the process yield of semiconductor device.